1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and more specifically to a lithographic projection apparatus including a radiation system for supplying a projection beam of radiation, a support to support patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table to hold a substrate, and a projection system to project the patterned beam onto a target portion of the substrate.
2. Description of the Related Art
The terms xe2x80x9cpatterning meansxe2x80x9d, xe2x80x9cpatterning structurexe2x80x9d or xe2x80x9cmaskxe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as herein set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithographic apparatus, the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include laser-produced plasma sources, discharge sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron radiation sources and condensers for projection x-ray lithographyxe2x80x9d, J B Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).
Other proposed radiation types include electron beams and ion beams. These types of beam share with EUV the requirement that the beam path, including the mask, substrate and optical components, be kept in a high vacuum. This is to prevent absorption and/or scattering of the beam, whereby a total pressure of less than about 10xe2x88x926 millibar is typically necessary for such charged particle beams. Wafers can be contaminated, and optical elements for EUV radiation can be spoiled, by the deposition of carbon layers on their surface, which imposes the additional requirement that hydrocarbon partial pressures should generally be kept below 10xe2x88x928 or 10xe2x88x929 millibar. Otherwise, for apparatus using EUV radiation, the total vacuum pressure need only be 10xe2x88x923 or 10xe2x88x924 mbar, which would typically be considered a rough vacuum. Further information with regard to the use of electron beams in lithography can be gleaned, for example, from U.S. Pat. No. 5,079,122 and U.S. Pat. No. 5,260,151, as well as from EP-A-0 965 888, which are incorporated by reference.
Working in such a high vacuum imposes quite onerous conditions on the components that must be put into the vacuum and on the vacuum chamber seals, especially those around any part of the apparatus where a motion must be fed-through to components inside the chamber from the exterior. For components inside the chamber, materials that minimise or eliminate contaminant and total outgassing, i.e. both outgassing from the materials themselves and from gases adsorbed on their surfaces, should be used.
For certain applications, a gravity compensator is required to exert a bias force that at least partially counteracts the weight of an object to be supported and which compensator largely prevents transmission of vibrations in the support direction. The height of the object may be varied by motors, and the application of a gravity compensator relieves the motors of supplying a force to overcome gravity, leading to a considerable reduction in power consumption and heating of the motors. However, such gravity compensators having a pneumatic working principle are known, but their application in a vacuum environment is presently not feasible, since the possible escape of gas from the gravity compensator would seriously disturb the vacuum. To prevent transmission of vibrations in a horizontal direction, it is known to provide a supporting means, such as a pneumatic gravity compensator, with a horizontal air bearing. Gas flowing out of the horizontal gas bearing will seriously disturb the vacuum. Above supporting means are described in EP 0 973 067, which is incorporated herein by reference.
A lithographic projection apparatus in accordance with one aspect of the present invention includes a radiation system for providing a projection beam of radiation, a support structure to support patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table to hold a substrate, a projection system to project the patterned beam onto a target portion of the substrate, and a supporting structure comprising a support member having a finite stiffness in a perpendicular direction that is substantially perpendicular to a support direction of the support member.
In a further embodiment of the present invention, the apparatus further comprises a vacuum chamber having a wall enclosing the support means, wherein the supporting means further comprises a gas-filled pressure chamber, the gas in the pressure chamber acting on a movable member such as to at least partially counteract a force substantially parallel to the support direction, and evacuating means constructed and arranged so as to evacuate gas escaping towards the vacuum chamber through a gap between the movable member and a bearing surface.
According to yet a further aspect of the invention there is provided a method of manufacturing a device including providing a substrate that is at least partially covered by a layer of radiation sensitive material, providing a projection beam of radiation using a radiation system, using patterning structure to endow the projection beam with a pattern in its cross-section, projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material; and
providing an isolated reference frame characterized by supporting one of the support structure, the substrate table and the isolated reference frame with a support member having a finite stiffness in a perpendicular direction that is substantially perpendicular to a support direction of the support member.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.